Understanding the Inverse Square Law in Photography.

Don’t let the title of this post scare you away! The inverse square law is actually quite simple and understanding it is one of the keys to understanding photographic lighting. Simply put, if you double the distance between a subject and a light source, then the illumination on the subject will be decreased by 4 times. Inversely, if you halve the distance between a subject and a light source, the illumination will be increased by 4 times.

The thing to realize here is that the relationship between distance and illumination is exponential. Illumination fades away very quickly as you move a light away from a subject.

Let’s see this in images. In each shot the camera is set to f8 at 1/500 and iso 100.

1. 70cm, f16                 2. 140cm, f8                   3. 280cm, f4                 4. 560cm, f2

In the first image the light source is 70cm away from Darth Maul and the light meter reading is f16. In each photograph, the distance between the light and Maul is doubled. So you see that when the light on Darth Maul is moved from 70cm away to 140cm away, the light meter reading changes by two f stops (16 down to 8). So even though we doubled the distance, the amount of light is reduced by 4 times (-1 stop is half the light, -2 stops is 1/4). This is where the “square” part of the law comes into play. The second photo is 2 times further away than the first. 22= 4. The inverse of 4 means that there is 1/4th of the light than in the original. The 3rd photo is 4x further away 42= 16, so 1/16th the amount of light. The lighting in the last photo is 64x darker than the first.

It is useful to understand the inverse square law, because it is one of the easiest ways to control the amount of light you use. If you find yourself in a situation where your light source is to dim, simply move the light or the subject half the distance closer. This will give you 4 times the amount of light (or two more f-stops) to work with.

One more noteworthy point about this law is that you can’t apply it to the sun. Anywhere on earth is relatively the same distance away from the sun, meaning you can’t get closer or further away to the sun to change the amount of light in your photograph. A workaround for this problem is to use a reflector. When you use a reflector, you are using the sun to create a new light source that is closer to the subject.

Now, go outside and use the Inverse Square Law!!!

Viewfinder Center for Photography is offering a class on Natural & Available Lighting on April 18, 2009 which covers this topic in depth as well as much more to help you understand and use lighting to create better photos.